A useful method of preparing an aldehyde starting from a carboxylic acid is the so-called Rosenmund reduction in which the acid is converted into an acyl chloride which is reduced with hydrogen to the aldehyde.
In European Patent Application No. 101,111 a process for the preparation of aldehydes is described in which certain carboxylic compounds are directly hydrogenated to the corresponding aldehydes over a catalyst comprising at least one rare earth metal and at least one additional metal, e.g., iron, on a carrier. The quantity of the addition metal(s) amounts to 0.1 to 20%w and preferably 0.5 to 10%w, calculated as the element(s) and based on the weight of the carrier. The catalyst shows a rather good reactivity and a fair selectivity to the aldehydes.
However, it appeared that during the hydrogenation of a carboxylic acid over the above catalyst a considerable loss of the rare earth metal(s) took place. This loss is probably due to the formation of rare earth metal salts of the carboxylic acid to be hydrogenated and the subsequent sublimitation of the salts thus obtained.
It has now been found that iron oxide can be used as a catalyst in the preparation of aldehydes, provided that the catalyst comprises a considerable quantity of iron oxide. The presence of a rare earth metal is not required. Such a catalyst can very suitably be used in the hydrogenation of aromatic carboxylic compounds and aliphatic carboxylic compounds having at most one .alpha.-hydrogen atom in the acyl group.